It's called 'throw-away ticketing'. Officially you can get in big trouble for this, because you're violating the 'contract' you have with the airline when you bought the ticket.

However, IF you get caught, you would be responsible for the fare difference in the ticket (I would pay by cash if I were you). You could be penalized by losing any FF miles you would've gotten as a result of this purchase, and the possibility you would lose ALL of your FF miles in your account.

If you don't have a FF account with the airline in question, I wouldn't worry about it too much.

Hey, don't worry. People do this all the time. You will not get credited any points or miles for the flight that you where not actually on, but in my experience (which is not unsubstantial, as this is what I do for a living), you have nothing to fear. We always appreciate if you could give as a call after arrival and let us know that you won't use the return. If it makes you feel better, you can make up an excuse why, but don't worry, this will be fine.

No airline is going to penalise any passenger for not showing up for the return sector. Don't worry about getting caught. The airline is not going to spend money on an investigator trying to hunt you down. They will not even put you in their black book either. Airlines want people to fly with them not scare them away. So let's all enjoy this bonanza before the alliances turn the clock back to the pre-deregulation days.